Microlithography is used for producing microstructured components, such as for example integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination device and a projection lens. The image of a mask (reticle) illuminated by the illumination device is in this case projected by the projection lens onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In projection lenses designed for the extreme ultraviolet (EUV) wavelength range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process. Such EUV mirrors have a mirror substrate and a reflection layer system for reflecting the electromagnetic radiation impinging on the optically effective surface.
In order to avoid damage to the chemically reactive layer materials of the reflection layer system by way of EUV radiation incident during operation, it is known, inter alia, to apply a capping layer onto the reflection layer system, which capping layer can be produced from e.g. a metallic material or an oxide or nitride and which can contribute, inter alia, to suppressing a diffusion of e.g. oxygen (O2) from the adjacent gaseous phase into the reflection layer system.
A problem occurring in practice, in particular during transport, storage or operation, is that such capping layers are also susceptible to molecular contamination (e.g. by hydrocarbons), wherein corresponding contamination depositions on the respective mirror may lead to an impairment of the reflection properties thereof and hence to a reduction in the performance of the projection exposure apparatus overall.
In respect of related background art, reference is made by way of example to US 2003/0064161 A1, WO 02/054115 A2, US 2007/0283591 A1, U.S. Pat. No. 8,764,905 B1 and U.S. Pat. No. 8,742,381 B2.